Thanks to their large area emission, ultra-thin and flexibility device architecture, organic light-emitting diodes (OLEDs) can be used in a wide range of trend-setting applications such as displays. Due to their robustness against environmental influences, quantum dots (QDs) based on inorganic semiconductors offer an alternative to the organic dye molecules. Due to their synthesis from the liquid phase, QDs can be used in applications independent of their substrate size with simple printing or coating processes. The emission wavelength of QDs can be tuned by the composition of the material and by the size of the QDs due to confinement effects. Similarly to OLEDs, support layers are required for efficient light emitting devices (LEDs) on the anode and the cathode side to support the injection of charge carriers into the emitter layer (QDs). However, industrial application of QD based LEDs is still in the beginning compared to the OLEDs due to the usage of materials which are harmful to the environment (e.g. Cadmium (Cd)) and / or limited in supply (e.g. Indium).
The focus of this work is laid on the development of efficient and bright solution based QD-LEDs. Based on the establishment of state-of-the-art process and measurement technologies in the department, the organic support layers in known QD-LED systems based on cadmium based core/shell QDs were optimized. In order to replace transparent indium tin oxide (ITO) electrodes, graphene was implemented as a carbon based alternative. In a systematic investigation of the effect of the number of graphene layers on the device behavior, an electrode based on a bilayer of graphene even achieved higher light output than the ITO reference.
Zinc oxide nanocrystals (ZnO-NC), whose working mechanism as an inorganic support layer is discussed controversially in the literature until today, was introduced to increase the efficiency of the QD-LEDs. By comparing the device performance of QD-LEDs with different ZnO-NCs with simulations, an explanation for the ZnO’s function as a hole blocker or electron injector was given for the first time.
Based on the findings on the Cd-based layer system, the integration of Cd-free QDs made from copper indium sulfide (CuInS2) could be realized in a QD-LED. With the help of the specifically engineered ZnO-NC support layer, the external efficiency of the devices was increased by three orders of magnitude to 0.4 % and the brightness by the factor 50 to 110 cd/m². Due to the broad emission spectrum of the CuInS2-QDs with ca. 100 nm for an emission wavelength of ca. 595 nm, the QDs could be used as a basis for white light emitting QD-LEDs in combination with blue QDs in an innovative mixed layer architecture. In this work, white light emitting QD-LEDs with a record color index (CRI) of 78 with only two emission components were achieved, which had a stable light emission for a wide voltage range without a large color drift. This is an important step towards the application relevance of cost effective, solution based, and sustainable QD-LEDs.

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